I have just as an experiment. 70% was too high. Also, it's very tough to control the movement for 10s precisely…..it's very easy to get to the low point at 8s or 11s. When the time duration gets that long it seems to be a little more difficult.
I didn't really like the effects of it. It seemed to have a decent metabolic affect but this was outweighed by the soreness that accompanied it. I prefer to use heavier loads and shorter times when working on eccentric strength.